The present invention relates to a switching type DC/DC converter which converts a dc voltage to another dc voltage.
Of control systems each used for a switching type DC/DC converter, feedback type PWM (Pulse Width Modulation) control has been used most commonly. A ratio between on and off times of a switching element can automatically be determined according to the value of an input voltage by making good use of a feedback function. An output voltage can hence be made accurately coincident with an expected value.
However, since it is necessary for feedback control to use an amplifier high in gain, this offers a possibility of oscillations. Since oscillation conditions change depending on an output load current, an output load capacity and the like, it is necessary to carefully add an oscillation prevention circuit according to the conditions of use. This is very hard for users unaccustomed to the prevention of oscillations to use.
As another problem, there is a point that since the output voltage is set by feedback, it takes time until the ratio between the on time and the off time reaches the optimum value. It therefore takes time to reach a stable state when the input voltage and the output voltage vary.
PFM (Pulse Frequency Modulation) control based on a feed forward system with no feedback has also been in widespread use to improve sensitivity. In a DC/DC converter described in, for example, Japanese Unexamined Patent Publication No. 2005-218166, a switching element is not driven during a period in which an output voltage exceeds a predetermined target voltage. When the output voltage becomes lower than the target voltage, the switching element is brought to an on state. Energy is stored in an inductor while the switching element is in the on state. When the current flowing through the inductor exceeds an upper limit value with time, the switching element is brought to an off state for a prescribed time.
A problem of the feed forward system resides in that in the case of a light load, a ripple voltage becomes large and power conversion efficiency is hence degraded. In order to cope with this problem, the DC/DC converter described in Japanese Unexamined Patent Publication No. 2005-218166 changes the upper limit value of the inductor current in reverse proportion to the switching period of the switching element.
A technology that changes an upper limit value of an inductor current with the same aim as Japanese Unexamined Patent Publication No. 2005-218166 has been disclosed even in other documents. In a DC/DC converter described in, for example, Japanese Unexamined Patent Publication No. 2005-218167, an upper limit value of an inductor current is made high when the number of load systems is increased, whereas the upper limit value of the inductor current is made low when the number of the load systems is decreased. In DC/DC converters disclosed by H. M. Chen, et al., an upper limit value of an inductor current is changed according to a variation in output voltage (refer to H. M. Chen, D. D. Jiang and R. C. Chang, “A Monolithic Boost Converter with an Adaptable Current-Limited PFM Scheme”, 2006 IEEE Asia Pacific Conference on Circuits and Systems (APCCAS), December 2006, pp. 662-665 and H. M. Chen, R. C. Chang and P. S. Lei, “An Exact, High-Efficiency PFM DC-DC Boost Converter with Dynamic Stored Energy”, Proceedings of the 15th IEEE International Conference on Electronics, Circuits and Systems (ICECS), August-September 2008, pp. 622-625).